The purpose of this project is to determine whether dualtropic (MCF) viruses of AKR mice play a critical role in development of spontaneous leukemias in this strain. A set of new AKR MCF viruses of differing leukemogenic potential will be analyzed. Diversity among these isolates will be examined by oligonucleotide mapping of virion RNAs. The structures of the viral env gene products produced in fibroblasts will be compared in tests with monoclonal antibodies and by sensitive peptide mapping procedures. The unusual intracellular processing of these proteins will be studied. Analogous studies will be performed on MuLV glycoproteins expressed in normal and preleukemic thymocytes and spontaneous leukemias of AKR mice. These studies will probe the basis of specific lymphoid cell phenotypes exhibiting high levels of MuLV protein production, possibly of MCF type, and low levels of virion formation. In particular, the role of endogenous MCF-like genetic information in the preleukemic amplification of MuLV protein expression will be assessed. Profiles of viral messenger RNAs in these lymphoid cells will also be obtained and compared with mRNAs in fibroblasts infected with MuLVs of different tropisms. New antigens specific to AKR leukemias will be sought, by the use of antisera raised in mice and rats against leukemias transplanted from AKR or induced directly with AKR MCF viruses. The chemical nature of a new MCF virus-related antigen, G(AKSL2), will be determined.